In the known art said synchronisation mechanisms are provided with at least one stiffening spring which, positioned between backrest and seat, is aimed at opposing a certain strength to any rotation action of the backrest with respect to the base, exercised by the user. The stiffness of said spring, usually a helical spring, can be adjusted via means that allow the user to move an anchoring point of the spring with respect to its other anchoring point, thus varying the length of the spring when operating, and therefore the elastic response of said spring to the stresses exerted by the user.
The patent application EP 0.956.793 A2, in the name of the Applicant, relates to the use of a synchronisation mechanism for chairs or armchairs comprising a stiffening spring positioned between the base and the backrest frame of the chair or armchair, and means for adjustment of the stiffness of the spring provided with a movable anchoring striker element for the end of the stiffening spring engaged with said base. The movable striker element is linked in a sliding manner to the base of the chair, or armchair, and has a slanting surface designed to engage with a wedge, also sliding, but in a transverse direction with respect to the sliding direction of the above-mentioned movable striker element. The wedge, which can be activated by the user by rotation of a control stem positioned at the front of the seat, determines, due to its slanting surface, translation of the movable striker element which, in turn, defines the movement of the related end of the spring with respect to the other end, thus varying the length of the spring during its use.
Although extremely effective, said means for moving an anchoring point of the spring are mechanically complex and therefore involve a certain difficulty in production and assembly, with consequent high production costs of the chair or armchair.
The international patent application WO 02/058514 A1 describes an adjustment mechanism for a chair of the type in which both the seat frame and the backrest frame are revolving, in a synchronised manner, with respect to the base of said chair, and in which two compression springs, positioned between base and backrest frame, provide an adjustable strength to the rotation of said backrest frame with respect to the base. The device for adjustment of the stiffness of the springs is fitted on the backrest frame and comprises a cam shaped element, which can be rotated by the user, which engages with a connecting rod hinged, at one end, to the backrest frame and also linked, at the other end, to the ends of the springs acting on the backrest frame. The rotation of the connecting rod with respect to its hinge point, determined by the rotation of the cam element, causes variation of the length of the springs when at rest, and therefore entails variation of the stiffness of said springs.
Unlike the solution proposed in the application EP 0.956.793 A2, in which the rotation of the backrest frame with respect to the base can involve a minimum rotation of the spring with respect to the spring stiffness adjustment device, which is fixed on the base, with possible wear between the parts, or accidental variations in the coupling position between movable striker element, adjustment wedge and end of the spring, the arrangement of the spring adjustment device on the backrest frame described in WO 02/058514 A1 would appear to guarantee that there is no wear or variations in the reciprocal arrangement of the springs and adjustment device, given the substantial identical rotation of the adjustment device and stiffening spring.
Nevertheless, in the mechanism illustrated in WO 02/058514 A1, the user activates the cam element by means of a revolving control stem which, being arranged at the rear of the seat, is difficult for the user to access.
The aim of the present invention is therefore to produce a synchronisation mechanism for a chair or armchair that solves the problems of the prior known art and which is therefore provided with a device for adjustment of the elasticity of the stiffening spring which can be easily produced and easily operated by the user.